1. Field of the Invention
The present invention relates to a wireless communication system for transmitting and receiving a radio signal, a transmission terminal and a reception terminal that form part of the wireless communication system, and a transmission method executed by a transmission terminal.
2. Description of the Related Art
Wireless communication systems have advantages of eliminating burdensome cabling that is required in wire network systems and providing flexible arrangement of terminals. Therefore, the wireless communication systems are now widespread for business use and household use.
Wireless communication systems conforming to the IEEE802.11a/b/g standards or the like, which are widespread as a wireless Local Area Network (LAN), use the Industrial Scientific and Medical (ISM) frequency band which is permitted to be used without a legally-required license. Therefore, a wireless terminal other than the LAN terminals can use the same frequency band as that used by the LAN terminals. However, in such conventional wireless communication systems, even if a plurality of wireless communication systems using the same frequency band coexist, countermeasures for the coexisting systems are not considered usually. Therefore, if a plurality of wireless communication systems using the same frequency band coexists within a common communication region, collisions between radio signals transmitted from the wireless communication systems occur frequently and interferences between radio communications also occur, which leads to occurrence of communication errors.
If the same kind of wireless communication terminals performing the same access control (e.g., wireless communication terminals belonging to the same wireless communication system) coexist within a common communication region, each of the wireless communication terminals analyzes a radio signal transmitted from the opposing terminal and adjusts transmission timings so as to minimize the likelihood of collisions between the radio signals. In contrast to this, if the different kind of wireless communication terminals which use different communication protocols (e.g., wireless communication terminals each belonging to different wireless communication systems) coexist within a common communication region, each of the wireless communication terminals does not adjust transmission timings for avoiding collisions between radio signals, thereby interference between the radio communications occurs.
One of countermeasures for avoiding collisions of radio signals is, for example, to select a radio frequency band, which makes the interference between radio communications less than a predetermined reference level. Concretely, when the wireless communication system is configured, the system measures a peripheral radio wave environment and selects a radio frequency band to be used on the basis of a result of the measuring so that the interference between radio communications becomes below a predetermined reference level. However, since the peripheral radio wave environment sometimes changes even after the wireless communication system has been configured, countermeasures for such change of the radio wave environment (e.g., the wireless communication system monitors the interference status of radio communications and adjusts the radio frequency band to be used on the basis of the result or the monitoring.) are necessary.
A wireless communication apparatus for avoiding mutual interference between wireless communications is disclosed in Japanese Patent Kokai Publication No. 2003-37529, for example. The wireless communication apparatus disclosed in this document tries to prevent the mutual interference between radio communications by interrupting the radio communication at a timing when an intensity of the radio signal received in the wireless communication system becomes higher than a predetermined reference intensity (e.g., when the apparatus determines that another wireless communication system perform a radio communication).
For example, in the case that the wireless communication system is a video transmission system which requires transmission of a radio video signal at specific timings, the video transmission system needs to transmit a radio video signal at specific timings even if a radio signal transmitted from another wireless communication system is detected. However if the video transmission system interrupts the radio communication if another wireless communication system initiates another radio communication in the similar manner to the conventional wireless communication apparatus disclosed in the above document for the purpose of avoiding the mutual interference between radio communications, a problem arises that the reliable radio transmission of the video signal cannot be implemented.